Abstract: Rainfall and reservoir water are the most common factors inducing shallow landslide deformation. For large expansive soil landslides, the combined effects of rainfall and reservoir water on deformation and failure mechanisms are often overlooked. In order to effectively explore the deformation mechanism of large expansive soil landslide under the combined influence of rainfall and reservoir water, this paper takes Jingang village landslide in Yunyang District, Shiyan City as a case study. Based on a comprehensive analysis of rainfall, reservoir water level fluctuations, and GNSS monitoring data, combined with field geological exploration data and macro inspection information, numerical simulation was conducted for the Jingang village landslide in the Danjiangkou Reservoir area. The simulations considered reservoir water level fluctuations, rainfall, and multi-factor interactions. The results indicate the following: (1) The Jingang village landslide exhibitis an overall slow deformation trend, with rainfall having a significantly greater impact on landslide deformation compared to reservoir water fluctuations. (2) Rainfall and reservoir water levels show different response modes in influencing expansive soil landslides. Heavy rainfall plays a dominant role in triggering landslide deformation and failure, while reservoir water level fluctuations have a relatively smaller impact, primarily affecting the landslide’s front edge. The combined influence of rainfall and reservoir water levels results in more pronounced stability damage. (3) Continuous deformation of expansive soil landslide under the combined influence of rainfall and reservoir water is affected by external factors and reduced internal strength.